text:
Is there an API to retrieve airport departure and arrivals?
I am looking for an API to get the arrivals and departures timetable for any airport in real time, like this table on the EuroAirport website:

Are services like FlightAware or FlightStats relevant for me? I am a non-native English speaker so airport-specific language is a bit tough for me, I am not sure I fully understand what those APIs provide.
Edit : paying APIs are okay, I have a budget of around 100€ a month for this project

While some of us are programmers, your probably better served on stackoverflow.

@Peter that question would be very off topic on stackoverflow. Questions asking us to recommend or find a book, tool, software library, tutorial or other off-site resource are off-topic for Stack Overflow <-- close reason

I found answers about FlightAware on Stack Overflow but they are not really clear nor provided by aviation experts, that's why I try my luck here :)

Same here? Stackoverflow is full of this questions and answers, because the people need answers for this kind of questions.  @Eric Burel: Does FlightAware not offer the required data?

Maybe this, looks like the data is not free of charge:   https://developer.flightstats.com/api-docs/scheduledFlights/v1

Not any airport, but is it free of charge for limited use and seem to be relative straight (no weird XML -> good):  https://developer.fraport.de/store/documentation

Have you check this out? https://flightaware.com/commercial/flightxml/

It appears that Flightaware does have an API (both SOAP and REST) that would satisfy your needs.
Here is the documentation, see "arrived", "departed", "en route", and "scheduled".
Here's a code example from that page that gets data on flights en route to JFK and KSMO (this fits your arrivals criteria no?):

That said, you'll probably want to experiment with the API once you create an account and get a key to see if it meets all of your requirements.

Thanks for this answer and all the comments. I indeed think FlightAware will be okay, I'll let you know when I have tried it.

You could probably try with ADS-B exchange. You can query their server using the following url https://public-api.adsbexchange.com/VirtualRadar/AircraftList.json
It will give you a list (in the form of a json string) of all the aircraft currently being tracked (the "acList" tag contains the list of aircraft). Most aircraft operating in major airports are equipped with ADS-B transponder so they would appear in your feed.
You can of course filter the json string around your area of interest. By reading the documentation, it looks like appending ?fAir=LHR would give all the aircraft currently tracked that departed from/en route to Heathrow.
It should be pretty easy to then parse the string to get the information you want (carrier, type of aircraft etc.). All programming languages these days have libraries allowing to easily and quickly parse json. I am not sure how often the server refreshes it's data...every second maybe?
Not the most practical solution, as I am not sure if there is an easy solution to work out ETA or departure time but you could give it a try (it's free!)

But this is not what OP asked for. Using ADS-B does not provide that info.


text:
Does an Airbus A320 Navigation Display depict constraints for flightplans other than the primary?
When any other flightplan (Missed Approach, Alternate, Temporary, Secondary) except the Primary flightplan is depicted on the ND in an Airbus A320 and the CSTR (constraint) button is pressed on the EFIS, will constraints for both flightplans be displayed on the ND?

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sort: votes


text:
What do the abbreviations on this Jeppesen Dataplan mean?
Attached is a screenshot from a flightplan created with Jetplan. Can someone help with decoding the format? What do the abbreviations mean in the table header?

I understand the following:
FLT = Flight Level *100
T = ?
WIND = Wind speed & direction
TAS = True Air Speed
S = ?
GRS = ?
MCS = ?
DST = Distance to next waypoint
DSTR = Distance remaining
ETE = estimated time en-route
ETR = estimated time remaining
FU = fuel used?
FR = fuel remaining?
FF/E = ?

GRS = Groundspeed
MCS = Magnetic Course
FU = Fuel burned that leg (100s)
FR = Fuel Remaining
T = OAT in degrees C, apparently without the sign.
S = Shear value (measure of turbulence) forecast
FF/E = Fuel Flow per Engine per hour
With thanks to @mins for filling in the gaps!

I doubt T is temperature. Is too high at those altitudes, even if it was in Fahrenheit.

@Stefan  Standard temp at FL180 is -21C, so 25 as temperature without the sign might be right.  This flightplan doesn't look to be in the stratosphere, just at 180.  The "S" value seems to always be a 1, unless it's omitted. No guess there.

that makes perfect sense, thank you to both @RalphJ and @mins!

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text:
Why are early wing planforms mostly rectangular?
I am looking at the history of wing planforms, and I am having trouble finding reasons for why the wright flier and other early aircraft were built with rectangular wings. I thought it was due to being more structurally secure, but I can't find any sources that explicitly say that, or describe the thought process of early designers.
Is my reasoning correct, and are there any other reasons for the decision to use a rectangular wing (vs a tapered wing)?

I would assume a square wing is much simpler to produce. The design and construction techniques required for a tapered or swept wing would be out of reach for such an early aircraft as the Wright Flyer. Additionally, the benefits of a swept or tapered wing are at higher speeds, and may not even be noticeable at the speeds which early aircraft operated.

I always thought taper was specifically for the lift efficiency of the wing due to less lift at the tips of the wing. Is that more relevant at high velocities (regardless of what early engineers knew)?

Actually, it may be more relevant at lower speeds and higher angles of attack, as far as drag, and more relevant at higher speeds for wing loading and roll rate. Taper is seen in gliders.  The story may be something like this, observed as a natural phenomena by the Wrights, understood and described by Kutta-Joukowski, masked by struts and wires in the biplane era, foreseen for use by Prandtl and Munk, applied in the 1930s as higher speeds and mono wings were developed.

The benefit of tapered wing lies in its proximity to the elliptical lift distribution while retaining much of the structural benefit of a rectangular wing. But we owe this knowledge to a few things:

The Kutta-Joukowski Theorem: published in 1906 by Nikolai Y. Joukowski and influenced a great deal by Martin W. Kutta.
The Lifting Line Theory: published in 1919 by Ludwig Prandtl and inspired by the work of Frederick Lanchester. The term induced drag was coined by Max Munk, a colleague of Prandtl in 1918. Although Lanchester had published some results on aspect ratio and finite-wing aerodynamics as early as 1907, they weren't taken seriously by his compatriots and had very little impact as a result.

The Wright Flyer flew in 1903. By all accounts, the Wright brothers designed it through trial and error. As a result, the insight of tapered wing couldn't have been known to them during the Flyer's design and through much of the First World War.
In any case, most of the early airplanes were bi/triplanes with external struts being their structural members. The drag from the struts would've overwhelmed any benefit gained from tapered wings.

The Wrights had a wind tunnel and extensive experience designing and flying their designs, even before considering adding an engine and going powered.  They certainly did not need Kutta, Joukowski, Prandtl, or Munk to tell them a rounded trailing edge did not pull as hard in the wind.  Their biggest mistake was a 2 year hiatus in building while they travelled and sold their "flying machine". But the ones who develop the theoretical understanding are certainly appreciated, as they are the true foundation builders of refining art into science.

Wing ribs are usually made with the aid of a template or tool. For metal wings, it could be the male and female parts of a mould used in a press. For wooden wings it's a board with blocks of wood that hold all the pieces in place while the glue dries. Either way, a rectangular wing only needs one tool for each rib, while a tapered wing needs a different tool for every two ribs (assuming you can reuse it for port and starboard wings)
In these days of CAD and CNC machining, it's easy to forget how much effort it took to scale drawings by hand, and build tools and templates that accurately matched those drawings.

A rectangle gives the maximum area for a given span, and for something like the Flyer, running on 12 hp, they needed all the wing area they could get in the lightest possible package, so rectangular it is.  Plus it's the easiest structure to build with minimum weight, since all the joints are simple 90 deg ones and the simple cross braced ladder structure is easy to make.  There are some (Fokker DVIII springs to mind), but overall you don't see many tapered wings prior to the 1920s.
The Wrights weren't engineers, just bicycle mechanics, but they really were geniuses.  They taught themselves all the math required.  They weren't really building on the work of others; they started out using Lillienthal's theoretical framework during their glider experiments, discovered that it was deeply flawed so as to be unusable, and built their own wind tunnel to work out their own numbers, to develop their wing from scratch.  On top of that, they designed and built their engine from scratch because nobody made one light enough.  Pretty amazing.

The rectangular "Hershey Bar" is a safe reliable, and yes, easier to build design.
But if you look more carefully at the Wright Flyer wings, you can see they were already rounding off the trailing edges to decrease drag. Tapering to a point serves the same function, and also enables one to build a bit more lightly due to reduced torque stress from the end of the wing.  Same reason a prop is usually tapered. It evens the load along the wing.
The common sea gull is a good example of tapering done nearly to perfection.
But aircraft mimicking this design lack the fine control gulls have of their wings and trade decreased drag for increased probability of often fatal "tip stalling".
"Washing out" (reducing) the wing tip angle of attack helps remedy tip stalling, as well as deploying slats at lower speeds.
The rectangular wing is fine for many recreational aircraft.  Another drag saving device that can be used is the Hoerner wing tip.  Basicly, it angles the bottom of the wing to more of a point when it meets the top, helping push the downflow of the vortex away from the top of the wing.
Tapering can be seen on gliders, and even on the Cessna 172, but it is not absolutely required in aircraft design if fuel savings or optimal glide range is not an issue.

"[...] reduced torque stress [...]" I think you mean bending stress, torque stress is about the spanwise axis. Also "But if you look more carefully at the Wright Flyer wings, you can see they were already rounding off the trailing edges to decrease drag" did they specifically give this as a reason? I'm not disputing it, but if you have a source I would like to read it.

F×d works either way.  The Wrights had a wind tunnel.  Their handiwork can be seen on the elevator too.  This is pretty basic stuff.  I marvel at the sea gulls.

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text:
What would happen if blades are not pitched correctly on a propeller blades?
What would happen on a 6 blade propeller if each blade opposite another was pitched in completely the opposite direction.

This seems entirely hypothetical, and we're focused on more practical matters.

Aren't they mechanically linked via some gearing in the propeller hub, thus that could never happen?

This document shows how a McCauley 2-blade propeller has its pitch controlled, 6-blade would likely be similar, but just more of it.  https://www.airtreknorth.com/uploads/4/7/2/4/4724302/von_klip_tip_cs_propeller.pdf

What do you mean by "completely opposite direction"? perpendicular? each blade 60° more pitched?

For a start, whoever built the propeller would get investigated for possible drug/alcohol impairment on the job.
In terms of trying to run the propeller, it would generate a lot of noise (from turbulence of the blades running in "reverse pitch", possibly stalled relative to the airflow induced by the previous blade) and very little or no thrust.  If it's on a piston engine (or electric motor, assuming a model aircraft or drone case), the engine/motor is likely to overheat after a short time due to lack of cooling airflow.
Finally, it's possible vibration induced by the noise-causing turbulence could lead to a structural failure of either the propeller itself, the engine/motor, or the engine/motor mounts.

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text:
how are UAV systems (aircraft+ground facilities) fitted with landing aid system?
For commercial aircraft, precise landing aid systems (ILS, GBAS) are required for autolanding, but these navigation systems are too expensive to be used for UAV operations. However, a lot of UAV OEMs have claimed and demoed landing the fixed wing UAV at a runway which is typically used for a Part 23 aircraft.

How do these fixed wing UAVs perform and autoland?
What kind of landing aided system are installed on the ground or on board?

UAV - U = unmanned, not necessarily unpiloted.

Similar to a Part 23 general aviation aircrafts, 1 to 5 tons

Each project will have its own specifications. Military UAVs will have different use cases from civilian ones, and, as the comments mention, unmanned does not mean unpiloted.
Most of current UAVs are remotely piloted, meaning that there is a front facing camera relaying images to the ground, where a pilot will command flight adjustments to the vehicle to keep it aligned, no special equipment involved (except for the air-ground communication apparatus). The aircraft will be equipped with the usual inertial sensor unit(s) and GNSS receiver(s).
For situations where you might have an unpiloted aircraft (that are extremely rare today), it is still solvable by simple INS + GNSS, with the additional requirement that you will have to accurately measure the position of the runway in the GNSS reference system (published charts are not precise enough, as they are created for eye-bearing humans, that can correct the course by sight). This is the method we used and we landed 2 meters from the centerline (the two pilots where there for safety, being it the first attempt).
Other projects might include ground-based aid signals akin to an ILS system.

2 meters, cool! So no ground landing aided system is used for the auto land and it is purely done by onboard navigation and control equipment? If it could be done by onboard equipment, then why  ground aided system like GBAS/ILS are must have to support auto land in the industry?

@YoungL because different situations require different levels of certainty. If the GPS fails during the landing, what would you do without ILS?

@YoungL because if a UAV runs off the runway, a few million $ is wrecked and you may close the runway for an hour or two for clean up. If a commercial airliner runs off the runway, it might be hundreds of millions in damage, plus hundreds of lives potentially at risk. With higher risk comes the necessity of higher safety margins provided by ground-based systems.

Many UAVs use specialized landing systems. For instance, the Boeing/Insitu Scan Eagle and Blackjack both "land" by snagging a vertical line with a wingtip mounted hook. To do that reliably, the Skyhook retrieval system has a GPS antenna at the top of that line and transmits its location to the incoming UAV which homes in on a location directly underneath the antenna. That it succeeds reliably despite wind turbulence, motion of a ship-board landing and other challenges and has for nearly 20 years strikes me as pretty amazing!

A don't know what systems they're using (probably classified). I do know however that military GPS has the "password" to decode the additional GPS frequencies unavailable for civilians, and this gives GPS a much precise location. That should be enough to land a UAV.
Civilians can't use this, but most of the absolute error is due to changes in the ionosphere. Relative precision is a lot better. So if you have a receiver at the airstrip, and you broadcast your location, the UAV can calculate its relative position to the airstrip a lot better. This must be good enough as well. Actually, GPS has around 3 meters (10 feet) of error in the open sky on flat ground - incidentally most airstrips has these features - and airstrips are quite big. Smaller UAVs could land based solely on GPS without external help on airstrips built for bigger planes, because that 3m difference is good enough.
(Those are based on how I would do it. I don't know enough about UAVs, but I have experience with different GPS systems.)


text:
Why do the ratio of proppeler plane over jet engine seems to have change in Bucarest airport during the covid19 crisis?
I live in a village very close to an (intl) airport: Bucharest "Henri Coanda" International Airport.
Now, due to lock-down caused by covid pandemic, I noticed - at least today when I spent most of the time outdoors - that all planes that flew over/around our location were propeller, none of them was a jet-plane.
Companies that normally operate flights on this airport: KLM, WizzAir, Alitalia, Delta, Aegean, Air France, Tarom, etc.
The main types of aircraft that normally operate on this airport are: A318, ATR42/72, B737, etc.
Do you have any idea why only propeller-planes these days?
I don't think it's a coincidence or just an impression, so I guess there must be some logical explanation.

Welcome to AviationStackExchange. You will find this site is mainly a fact based question and answer forum. Opinion based questions are frowned upon and frequently get down-voted and/or closed. Would it be possible to rephrase your question to include what facts your research has garnered so far. Info such as location, normal air traffic (type, company, make/model, etc.), flight pattern details, etc. will help others respond to your question. After all different countries will have differing experiences with Covid 19.

I would recommend asking something like, “What is the percentage of decline in commercial Passenger, Cargo, and General Aviation traffic for X[airport, city, and/or country]? Living under the flight path for a regional Bravo with an additional international Bravo and two extremely busy Deltas all within 20 miles of me, I have noticed more of a change in jet traffic than in light prop traffic.”

Thanks Dean, but I am interested in a qualitative answer, rather than a quantitative answer - I would rather want to know the cause, than a percentage. Please excuse my bad English...

No one can definitively know “the cause” without knowing “the where” (and maybe the who and what). I have regional jet pilot friends who are surprisingly busy. Cargo jet pilots will also be very busy. There are flight schools who have completely shut down instruction in some locations but not others for the same flight school. There are charter companies still flying, and rental companies still renting. Not to mention aircraft owners still flying. Aviation is deemed an essential service in the US. At this time, it is not subject to current sequester orders.

Well, it's easy to replay the day in flightradar24 (only for 7 days for free; replay is the clock button on the right). There were some jet flights too, but there sure was rather many flights of C-130 and C-27J military transports. Since they were not scheduled, you'd have to watch them to see where they were going. And check local news for what government business they might have been up to.

I have no idea about Bucharest specifically, but at least in the US, many GA pilots are taking advantage of the significant reduction in airline traffic to fly into big airports they're normally not welcome at. And, for the most part, ATC is welcoming these GA flights because they're bored too.

If you're going to downvote, give a reason so folks can improve their answers.

that's true, i feel like people downvote for no reasons some times. Like if you don't have a better answer, then don't downvote! (Also I upvoted for you)

It depends on where you are located. In the US, vacation travel has dramatically dropped but not completely subsided. Business travel for the road-warrior/frequent-flyer types has dramatically dropped but not completely subsided. There are people who work in essential service operations that still require air travel. Cargo distributed by air has not declined. It may even have increased. Individuals and companies that own their own aircraft are still flying. Just not with quite the same frequency as before Covid 19. These aircraft are typically smaller aircraft that are prop or turboprop driven.
It could also be that a lack of air traffic has made any air traffic stand out. Think about the complete airspace shutdown during 9/11. Any military aircraft in the air was drastically more conspicuous. In the current Covid 19 environment, a normally scheduled air photography, air survey, utility/pipeline inspection, Part 141 pilot training, etc. flight will still go on as scheduled (depending on the company, client, and location). It will just stand out like the proverbial sore thumb.
Case in point, the DFW area is still very busy airspace. Just not as busy as before Covid 19. It may still be busier than it was during the economic recessions of the past 50 years. Aviation is economically driven. There are fewer pilot training flights during Covid 19. But, it has not stopped entirely. Especially with the number of foreign airlines sending their cadet pilots to the US for training prior to the pandemic. Plus, Texas has a plethora of airports (big, small, civilian, military, public, private, and mixed use). Small prop and turboprop aircraft can use any and all of them except private (except by permission) and military.
It may be helpful to check the registration of the aircraft still flying if it is possible where you live. This will give you more of an answer of why they are flying.

I think this is just the case for your airport because nowadays, it's actually the opposite for the big international airports. But, some international airports are actually mainly regional, even though they say they're international. So, that might be why.
Another possibility is GA planes. Some international airports still accept GA flights, even big hubs like Vancouver and Chicago!
But, there are benefits of having propeller planes. Firstly, they're more fuel-efficient, secondly, they're easier to fly in a pilots perspective, thirdly, they're easier to maintain, and much more.


text:
to whom are PAN PAN PAN calls addressed?
Can a PAN PAN situation be adressed directly to an ATC or must it be at adressed only at large?
This is a situation wereby the ATC was the destination before the situation of the pilote got worse.

Your second sentence does not make sense.  You don't fly to ATC, you communicate with Air Traffic Control.  Can you explain in a little more detail what you are asking about?  Is the question "to whom are PAN PAN PAN calls addressed" or something else? Oh, and Welcome to AVIATION.SE.  The [tour] and the [help] offer some guidance on how to get the most of an SE formatted site.

It's hard to understand what you're asking. Please add some more details

Really sorry. Actualy, the pilote is headed for the Montreal Airport and that is why he wants to comunicate with its ATC.

I beg for your patience, I do not know much about piloting a plane. Actually, I am presently correcting a novel that I have completed concerning a pilot of a CL-215 fire- fighting water bomber. It's a romance/aventure novel and I am trying to make it as realistic as possible.

Thank you very much. The information has provided the answer I was looking for.

Related

You may edit the question to add precision comments make you precise so that  it will be clearer for future readers of this question.

A PAN call is like a MAYDAY call in that you are broadcasting to whomever may hear it.  However, if you broadcast a PAN call on an active ATC frequency, ATC will naturally respond to it and it's sufficient to just state PANPAN-PANPAN-PANPAN and your registration and any other information and ATC will answer, and everybody else will know to stay off the frequency momentarily.
You can broadcast a PAN call on any other frequency like a unicom, or 121.5, and whomever is listening in will likely respond.  I was once downed in the bush in a seaplane by engine trouble, becoming stuck on a remote lake, and couldn't raise anybody on the normal unicom channel even if I heard other seaplanes passing in the distance.  I saw a contrail above, and knowing airliners monitor 121.5 over sparsely settled areas, I broadcast a PAN call with my registration on 121.5. To my surprise the airliner heard me and answered.  I was able to get him to contact base on the Unicom frequency and let them know what lake I was on before he was out of range of my radio's transmitter, and was rescued with my two fisherman passengers later that day.  The only other choice was to set off the ELT and hope an airliner or SAR aircraft heard it (this was pre-SARSAT), which would have meant spending the night there at least.

Thank you very much. The information has provided the answer I was looking for.

The correct phrase is "PAN-PAN PAN-PAN PAN-PAN" not "PAN PAN PAN". The "PAN PAN" needs to be repeated 3 times. From the Pilot/Controller glossary: "PAN-PAN− The international radio-telephony urgency signal. When repeated three times, indicates uncertainty or alert followed by the nature of the urgency."

Well I'll be darned.  I learned to fly in 1975 so I guess I'm a little out of date!

If you are on an ATC frequency, you are communicating directly to ATC while you are communicating at large.
Airband frequencies are not like CB (Citizens Band) frequencies. It is not a free for all. Anyone can listen. But, there are rules to follow if you are transmitting.
For instance, in the U.S., radio communications are not governed by ATC (Air Traffic Control) nor the FAA (Federal Aviation Administration). It is legally regulated by the FCC (Federal Communications Commission). They assign radio frequencies to entities for specific use. When they assign a frequency to ATC for an area (airport, airspace, Flight Information Region, etc), that entity effectively owns that frequency for that area. Others may use it for the express purposes of working with that entity with that entity’s permission.
Think of it like a house that you lease. You effectively own the house, driveway, and lawn for as long as you own the lease. People can not legally enter your property without your permission. Technically, they can walk across your lawn, but not legally. That’s called trespassing. You can have them prosecuted. Most people allow others to walk or drive on their driveway. But, only for the express purposes of coming directly to your front door for your purposes.
If you are communicating on the local frequency, your expectation is for the local entity to respond. If you want to communicate to a different entity, you have to change to a different frequency. Anyone may monitor those frequencies and hear your transmissions. But, they have no legal right to use a frequency owned by someone else except with the owner’s permission.
If ATC owns the frequency, generally, they only want you to communicate directly with them. Communication between planes is discouraged on ATC’s frequency unless it is used to relay messages between a far away plane and ATC or it’s used to communicate urgent information like an open cargo door or fuel leak visible on another plane. I’ve used this to communicate an attached tow bar on a plane crossing the runway hold-short line for takeoff.
Frivolous conversation between planes is even discouraged on frequencies the FCC designated as pilot-controlled. Keep all conversation pertinent to flight operations. Pilot-controlled frequencies are like public city streets or highways. They are owned by the city, county, state or federal government. The public can use them only for specific purposes governed by law.
During a Pan-Pan or Mayday transmission, ATC requires all other transmitters to remain silent unless they can offer assistance to ATC or if ATC addresses them directly. ATC will begin directing the non-emergency aircraft to different frequencies and airspace that will not conflict with the emergency.

Note that FAA and FCC are US authorities, but the OP said "Montreal Airport" in the comments, so they are probably in Canada.

@Bianfable - Good point. Thanks.


text:
How does Propeller windmilling work?
I’ve got a problem with understanding propeller windmilling. What I know is that when rpm are low and tas is high the blade aoa goes „over” the blade and is some kind of negative now, producing thrust in opposite direction than usual. But I don’t understand what really happens there. Is prop rotating in opposite direction or sth? I also read somewhere in the atpl book that when prop is windmilling the prop is driving the engine. How is that possible? Can somebody clarify that to me?

Do you know what a windmill is and how it works?  Really, I'm not trying to be a smart alec or anything, but go to a toy store and buy a little pinwheel and blow against it, or swing your arm through the air.  The force of the air makes it spin.  That is all that is happening with a windmilling prop.

Think of a windmilling propellor as like downshifting your car.  If you downshift (and back off the throttle), your car slows down because the wheels transmit force back to the engine, which drives the engine, causing it to speed up (that is, rotate faster) without producing more power.

You can think of the propeller as a screw. Take a screw, a screwdriver, and starts screwing. The threads of the screw drive the screw into the block of wood.
The propeller works in a same way: the engine drives the propeller that "screws" through the air.
It also works the other way around. Well kind of, with the screw, because of the high friction between the block of wood and the threads of the screw, but forget about the friction for a while, if you can.
If you stop twisting the handle of the screwdriver, and instead push the block of wood against the screw + screwdriver combo, what should happen? The screw should rotate just as if you were screwing it with the screwdriver, as the threads are still sinking deeper into the block of wood.  The screwdriver would of course rotate with it.
Again, it's the same with propellers (and engines). If you have speed, and you shut down the engine, the airflow will push against the propeller just as the (magically frictionless) wood against the screw. The propeller will keep rotating in the same direction, and as it does so, it keeps the engine turning also.
Now, in the case of you no longer twisting the screwdriver handle or the engine driving the propeller, there is no longer a force driving the screw or the propeller through the medium they are engulfed in. Instead, the medium (wood or air) is imposing a force on the screw & the screwdriver, or the propeller & engine to keep them turning.
If you do a google image search with words "propeller tip contrail" you'll find pictures displaying nicely how the propellers are actually screwing their path through the air.

Okey, I got it, thank you! One more question. Is there any „logic” explanation, apart from rpm&tas giving negative aoa why there is negative trust generated? Because „logicly” if the prop is rotating still in the same direction it is still repelling air backwards. This negative trust is generated because the acceleration of air is lower than the tas of aircraft and in fact the prop is stopping the aircraft or I’m wrong?

Negative thrust is just a fancy word for drag. It is the force that is imposed on the propeller by the air. When engine is turning the propeller, and it is pushing the air bacwards = thrust. When the air is pushing the propeller to keep it turning = negative thrust = drag.

Note that the fact that it is windmilling as opposed to being stationary, it's generating lift to rearward, which is much higher than a stationary blade just making turbulence.  If you stop a windmilling prop when gliding (which you can if you slow down enough) the drag of the stopped prop is way lower and the glide ratio improves quite a lot.

Rw "Because „logicly” if the prop is rotating still in the same direction it is still repelling air backwards." -- no.  The air is "hitting" the "front" side of the prop blade.  Like an airplane wing flying at a negative angle-of-attack and making negative (downward) lift.  In the case of the wing, you can say the airflow (wake) is actually being turned or curved "up" rather than "down", in the aircraft's frame of reference.  In the case of the prop, you can say the airflow past the propeller is being slowed rather than speeded up, in the a/c ref frame.

When windmilling, the propeller rotates in the same direction as when propelling the aircraft.

Picture above was also included in this answer, and shows the local Angle of Attack at the blade profile: a vectorial summation of rotational speed of the prop, and airspeed. As long as the AoA is positive, spinning the propeller by applying engine torque will create thrust. This is essential: torque rotates propellers.

Now stop applying engine torque to the propeller, and its rotational speed will decrease. Airspeed decreases much slower, so the AoA of the prop blade will decrease. At negative AoA, the incoming air flow will apply the necessary torque for turning the propeller, as depicted above right hand side. The windmilling propeller turns the same way.


text:
Is it possible to reload Electric Aircraft in mid-air from a tanker aircraft?
Can this be done military style refueling but with an electric wire boom supplying current from tanker aircraft to electric aircraft mid air in flight? Electricity from the tanker craft can be generated from its engines and/or an on board diesel generator. And utilising fast charging technology for quicker refuel times to recharge the electric aircraft's batteries.
Is a technology demonstrator already available? Or is Google not showing me any results?

Not exactly the same thing, but perhaps pertinent. Some work is being done charging drones with ground-based stations: https://newatlas.com/in-air-drone-charging-unlimited-range/56363/

Yes, drones or other light aircraft can be used as research for a start, but the whole idea is to do mid air refuelling to increase the electric aircraft's range

Considering that a Tesla Supercharger takes over an hour to fully charge a car's batteries. it would seem fairly pointless, since you'd have to have the refueling aircraft connected to the electric one for more time than it's not connected.

Another advantages is that when the docking cable connects to the electric aircraft, it can then take control of the motor controller controlling the propeller to sync it with the refuelling aircraft speed automatically without the electric aircraft pilot's intervention. Something that traditional refuelling methods can't. And remove the betteries from the power loop can make charging more efficient

That could be a lot of power going over those wires...

Shouldn't it be called "mid-air recharging"?

Charging an electric aircraft with a diesel generator - sorry, a flying diesel generator - sounds like an awesome environmental breakthrough! Just the kind the geniuses on Top Gear come up with all the time. Greta would approve.

@therac. A diesel generator could load share with the aircraft engines for more output power

How about using an extension cord?

I won't speculate about google's limit, at least not on this website

An automotive analogy is found in Formula E racing.  It never considered "refueling" through a power cable, instead opting for swapping entire cars mid-race.  Its parent company is now trying battery-swap stations for consumers, if not yet for the race cars.
So, replacing an airplane's depleted batteries with fully charged ones from the "tanker" might work. Although I shudder at how to engineer an airborne two-way conveyor belt, aerospace history is full of even more audacious challenges that have been overcome.

I hope battery swap in mid air were developed. It would provide noce pictures

I don't mind losing the accepted answer here, but how on earth is this one better @Nederealm Sorry CamilleGoudeseune , nothing personal against you ;)

Please note the pun is intended in the comment above :)

If munitions can be dropped from aircraft, then shouldn't be to much of a problem. The aircraft have to be in position belly facing top. The charged battery pack will be dropped into the electric aircraft. The empty battery bank can be returned to the tanker aircraft via some sort of propulsion system firing the cartridge upwards into the tanker craft. A switchgear should be able to disconnect the low battery and reconnect connect the new pack almost instantly

Dropping is easy, but catching is hard.  Once that's figured out, then after refueling, the tanker moves below the other airplane, which drops the empty battery into the tanker!  (You just need room, temporarily, for both batteries.)

This would certainly be possible, but the current status (pun intended) of the battery technology makes it pretty much pointless.
The charging would simply take too long: for example, Eviation Alice has a battery capacity of 900kWh. If we were to charge, say, 75% of this (because you would need to have some reserve capacity in case charging does not work out) it would take about 10 hours to charge if the charge time was comparable to Tesla Superchager (85kWh battery charges in about 75minutes =1.13 kwh/minute). Even if the battery was charged in ten parallel segments, charging would still take an hour.
Alice has an endurance of about two hours, so to double that, you'd have to spend at least an hour, most probably more charging.
Then, there's of course the sillyness of charging an electric aircraft with electricity made from jet fuel (or diesel)... it would be far more efficient to use that fuel to propel the aircraft without the engine-alternator-charger-battery-motor -detour.
For further reading, please see this Aviation Stack Exchange question regarding aerial refueling, many of the points raised in the answers also apply to aerial recharge.

Charging any battery takes about one hour. Larger batteries have more cells but they all charge simultaneously so it doesn't matter.

Well not any battery, but I have to say your point might very well be correct for most battery systems that would be feasible in aviation @RainerP.

If 70% charged is enough, then some batteries can reach that in ten minutes.  https://batteryuniversity.com/learn/article/ultra_fast_chargers

@CamilleGoudeseune a very good article, which also discusses alot about the limitations of ultra fast charging. As such, I do not see it matchin the ultra high safety standards of aviation :)

Tesla now has batteries to the point where you can recoup energy for a 4-hour drive in 30 minutes.  That's in full-on production,  you can buy it and do it at retail today.  The biggest challenge Tesla has is keeping the batteries cool during this slam charging; and in an airplane we have a huge surface area being blown by a slipstream.  Have the battery pack be a thin layer bonded to the underside of the wing, and you got that licked!
So I think we can safely say woth emerging tech, you could recoup energy for 40 minutes of flying in 5 minutes of tanking.
The problem is, this defeats the entire purpose.
You are spending between 4 and 10 times the fossil fuel to get the tanker airborne, on station and generating the recharge power, than if you just ran the e-plane on a gas engine in the first place. And wasting a whole second crew, and having to build the second airplane with positively ginormous auxiliary generators... The whole thing beomes an exercise in futility.
Use fuel for that mission.  Offset fuel burn elsewhere.
So you want an e-plane? OK, build a gas plane and buy the Postal Service ten electric jeeps.  Not only will it work better, with proven COTS tech, it'll be cheaper to boot.
The realistic picture for green aviation is to beat the Cessna 182 into feeder wires for high speed rail.  Let the electric truck deliver the epoxy, rolls of carbon fiber cloth, electric motors and battery modules.  Then, build something totally new, with new design factors from prop(s) to rudder.
Honestly, given the numbers Tesla is proving in production, I am optimistic about electrifying all sorts of things previously thought to be un-electrify-able.  I see 1200 mile flights with an electric plane to be perfectly feasible: fly for 4 hours, land, supercharge while you get breakfast, fly for 4 more hours, lunch, 4 more hours and home.

This question shows the fallacy of electric propulsion in its current state. The electricity needs to be generated using non-environmentally friendly means. Even solar power is unfriendly due to the process for making the current panels. Hydro and geothermal power would not work in this scenario. Even the production and disposal of the batteries have environmental consequences. All we are doing is shifting the environmental impact. At best, we are benefiting our immediate environment at the expense/ detriment of someone else’s in another place or time.
A better question is how do we use renewable energy to power a plane already in flight without the use of fossil fuels to create, transmit, or transport that power. Maybe with better, more environmentally friendly, and more efficient batteries, motors and solar panels, we could make the plane entirely self-sustaining.


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What happens if an aircraft tries to land at MTOW (if greater than MLW)?
Here's a scenario:
Just after Take-Off, there's an emergency on-board and pilot wants to land immediately. But, the weight of the plane wouldn't have decreased by large margins (usually 0.02*MTOW). Is the aircraft allowed to land with a weight more than Maximum Landing Weight?
What will happen if the plane tries to lands with a weight more than Maximum Landing Weight?

You might restrict your question to a specific type. Some short haul airliners may have a MTOW close (if not lower than) MLW.

If it "tries to land," what besides landing do you think might happen? The runway repels it so that it can't touch down??? I think what you're really getting at is, what happens after an overweight landing occurs, yes?

Most (all?) modern airliners are, in fact, certified to land at any weight up to MTOW if necessary, but if they do land at more than MLW then a special "Overweight/Heavy Landing Inspection" must be performed before the aircraft can be used again.
As this Boeing document states:

Overweight landings are safe because of the conservatism required in the design of transport category airplanes by FAR Part 25.
FAR criteria require that landing gear design be based on:
A sink rate of 10 feet per second at the maximum design landing weight; and
     A sink rate of 6 feet per second at the maximum design takeoff weight.
Typical sink rates at touchdown are on the order of 2 to 3 feet per second, and even a “hard” landing rarely exceeds 6 feet per second. Additionally, the landing loads are based on the worst possible landing attitudes resulting in high loading on individual gear. The 747-400 provides an excellent example. The 747-400 body gear, which are the most aft main gear, are designed to a 12-degree nose-up body attitude condition. In essence, the body gear can absorb the entire landing load. The wing gear criteria are similarly stringent: 8 degrees roll at 0 degrees pitch. Other models are also capable of landing at maximum design takeoff weight, even in unfavorable attitudes at sink rates up to 6 feet per second. This is amply demonstrated during certification testing, when many landings are performed within 1 percent of maximum design takeoff weight.


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What is the Dimension of Equilibrium Space in Longitudinal Space?
Full untrimmed Flight Dynamics model of an Airplane is separated into two separate systems: Longitudinal and Lateral.
When the untrimmed model in Longitudinal direction, which is six dimensional, is trimmed (f(x,u) = 0), what is the dimension of the equilibrium space? Is it two dimensional? If yes, then why?
x = state vector for longitudinal direction
u = control vector in longitudinal direction

When you say 6-dim, I assume you have included the elevator actuator and/or engine as separate states? If it's pure surface driven, then it would be 4-dim only.

Yes, Elevator and Thrust are different states. Here, x = [V, alpha, q, theta] and u = [Delta_Elevator, Delta_Thrust]. So, a total of 6 dimensions. I want to know what will be the dimension of equilibrium space.

The longitudinal differential system is:
$$\dot{\vec{\bf{x}}}=f(\vec{\bf{x}},\vec{\bf{u}})$$
where $\vec{\bf{x}}=[u,w,q,\theta]$, $\vec{\bf{u}}=[\delta_e,T]$
As far as linear space is concerned, control inputs $\vec{\bf{u}}$ are not states. So unless you have actuator models that construct internal actuator states, the longitudinal differential system is 4-dimensional.
As far as the equilibrium point is concerned, you set all the derivatives to zero, and impose a further restriction on $q=0$, $u=u_0$, then you have four unknowns (two state variables and two control variables) with four non-homogeneous algebraic equations. With linear aerodynamics, you have one unique solution, and it is not a vector space.

I never heard of the term "Equilibirum Space" in flight dynamics and I can't find references in a quick google search.
However, I assume you are talking about describing the flight dynamics of an aircraft as a linear state-space model. When linearising, you can indeed separate the equations of motion in the plane symmetry (longitudinal) from the assymetric (lateral) motions.
$\dot{ \vec{x}}=\mathbf{A} \vec{x} + \mathbf{B} \vec{u} $
The dimension of the state vector $\vec{x}$ is usually 4 (velocity, flight path angle, pitch angle and pitch rate), in your case the dimension of the control vector  $\vec{u}$ is 2 (thrust and elevator deflection).
You linearise the equations of motion around an equilibrium point; the state and control vectors describe the deviation from that equilibrium point. The dimension of the equilibrium points is 6 (4 state variable, 2 control variables).
One could say that the equilibrium point is in the space spanned by the 6 independent state and control variables. If that is what you mean, then dimension of the equilibrium space is 6.

I found about equilibrium space when I was reading a paper (DOI: 10.1109/ICCA.2019.8899603) on the control of an impaired aircraft. In the paper, a straight level flight trim plot had only 2 dimensional. I want to know why the dimension is 2.


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Which Velocity is used to measure Dynamic Pressure?
Suppose an airplane is flying in windy conditions. Now, for the calculation of the dynamic pressure should I use the relative velocity or the absolute velocity as the free stream airspeed?

A wing only cares about he airmass it is moving in. Thus you must take speed of the wing/the aircraft relative to the airmass you are flying in.

You should use the speed of the air relative to the object.
Take the extreme example of a stationary wind turbine. It obviously experiences aerodynamic forces and dynamic pressure, yet always has an absolute speed of zero.


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Is a retreating blade stall a problem that can be overcome on dual-rotor helicopters?
I know that regular main+tail rotor helicopters stall at higher speeds as the relative airspeed of the retreating blade approaches zero.
I'm curious if coaxial or intermeshing dual-rotor helicopters are affected by the same or a similar problem.
I'm aware that other factors may dominate and impose speed limits on them as well, but I'd like to know if a retreating blade stall is a problem that can be overcome with dual rotors.

Dual and intermeshing rotors also suffer from retreating blade stall at high forward velocity. But the effect of this is much less dramatic than with a single rotor.
Retreating blade stall causes a loss of lift on one side of the rotor: the single rotor helicopter will start to roll. A helicopter with two rotors that counter-rotate doesn’t experience this rolling moment, only an increase in drag.

So as I understand it, it can keep flying safely even if its efficiency suffers. Right?

@edgerunner Right.

Is it worth mentioning that in coaxials, the faster you go the upper retreating blade rises less while the lower advancing blade rises more, eventually leading to a very exciting intersection of the rotor cones?


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Is it possible to build 'Contra-rotating' EDF?
Is it possible to design an EDF (Electric Ducted Fan), with two counterrotating blades, that will have more thrust than two separate EDFs with the same blades?

Would the blades have to be exactly the same, or may we consider that they can be optimized for each case? I'm guessing you'd like the diameter to be the same in both cases.

And just a guess because I can: one EDF with two counter rotating blades and X kw of power would produce less thrust than two separate EDF's with the same blades and X/2 kw of power each. Because the version with two EDF's will have twice the area of prop disc and the independend blades are operating in undisturbed flow. Just guessing, though...

Related: https://aviation.stackexchange.com/questions/66400/what-are-the-relative-efficiencies-of-the-various-stages-of-a-counter-rotating-p

Please feel free to 'design' each fan independently.  Include a stator, etc.  The inlet duct would probably be ~30% smaller in diameter, at the throat, than the first fan.  The exhaust would also have a diameter, at the throat, about 20-30% smaller than the 2nd fan diameter.  Be creative!!

The primary reasons you'd want contra-rotating pairs with conventional (unducted) propellers are cancellation of torque/P-factor, and delivering more power with a limitation on diameter (tip speed, usually, or landing gear height).
Neither of these really applies to ducted fans; it's easy to null out the slipstream rotation with fixed vanes in the duct, and flow is always perfectly axial so there's no P-factor to worry about.  Further, pitch can be built high enough to absorb the needed power without supersonic blade tips, and generally is.
Bottom line, there's nothing to be gained from the extra weight and complexity of contrarotating fans in a duct compared to just using a slightly larger fan and motor to deliver the increased thrust you're chasing.

Probably not. In general, a stacked pair of contra-rotating propellers produces somewhere around 70%-80% the thrust of the same two props if mounted separately (Peter Kaempf can tell you the exact number). This is true regardless of whether the props are turned by electricity or gasoline.

I am not a turbine engine expert.  What I am trying to accomplish is, I think, something similar to a jet engine's compressor.  I want to increase the pressure ratio, which should increase the overall thrust.  But, I'm just guessing - - - -

BTW, thank you all for your comments.

Inline, contra-rotating EDF do exist in the RC community but they are very hard to get right. What does not exist are inline EDF configurations that produce more thrust than the two EDF units mounted separately.
There have been many failed attempts to get inline EDF to produce more thrust than a single EDF but there have been several successful attempts.
In case the above statement escaped your notice let me rephrase it: a lot of attempts putting two EDF units inline fail to produce significantly more thrust than a single EDF. My own attempts got me around 0% to 10% more thrust while using around twice the current. It's hard to get it right. But some people have managed to get more than that.
Of those who've successfully flown twin inline EDF units the advantage seems to be not extra thrust but higher overall efflux speed. Generally the rear EDF would have a higher pitch fan so it can accelerate the output of the first EDF without slowing it down. You will end up with a heavier plane with almost no gain in thrust but hopefully a higher top speed.
The main problem is if you get the pitch of the rear EDF wrong then it will simply act as an airbrake instead of an accelerator (you will sometimes notice this phenomena with pattern planes and motor gliders - non EDF - where diving with the prop spinning is slower than with the prop stopped).
The following are some of the more successful discussions/articles from various RC forums over the years. Unfortunately a lot of original content on the results of some of the projects are no longer online but the forum thread is still there:
https://www.rcgroups.com/forums/showthread.php?262808-Electric-Ducted-Fans-November-2000
https://www.rcgroups.com/forums/showthread.php?83207-Multi-stage-EDF
https://www.wattflyer.com/forums/showthread.php?t=67783
https://www.rcgroups.com/forums/showthread.php?1674106-Two-edf-units-in-the-same-duct

Non-inline staged EDF
There is one interesting experiment that used two regular EDF fans one behind the other but is not inline. Instead the first EDF is installed in a cheater hole that blows additional air into the intake from above the plane. This also did not get twice the thrust but got more than the usual amount of almost nothing. You can see the video of it here:


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Can anyone tell what kind or even which airplane this is?
As stated in this post on the Dutch Police website (Google Translate English version) this morning (12th june 2018), they are looking for all available information concerning the image below. All help is appreciated!

[Update] Additional information: the picture was presumably made on the 16th of January 2018 at 20:57 local time. They are looking for any information about airplane type, carrier or maybe even flight number.

Why do they want to know?   I don't see anything untowards there.

@Cees Would you contact Dutch police with all info gathered here?

@Bentoy13 since Cees did not respond and the information found seems legit, i've contacted the Dutch police and shared the information found so far

@CrossRoads Presumably, they found the photo on somebody's phone and want to know where the person was on that day. That could be in relation to a criminal inquiry, or a missing person case, for example. I edited the question to link the Google translation of the Dutch Police page.

Post it to airliners.net and they will be all over it.

Yes sorry for the late reply, thanks for all the help. I contacted the Dutch Police and notified them of this thread! Excellent work by Bentoy123!

@CrossRoads Here is an English article with some background.

It may be an A318 with cabine plan like this one for Avianca:

My guesses are:

the emergency light line stops around row 6, with the (red?) curtains around row 4.
the seats arrangement is approximately 3 rows of business class then economy class.
best guess would be 2x3 seats per row, with central screen every three row.
cockpit door is slightly not centered to the left

So it can fit with this type of plane.
Sorry,  I can't guess the company. The curtains seems red, but the cabine does not fit with AirFrance (and Hop) neither British Airways.
EDIT

According to the video posted by @Florian in comments, Avianca is a good guess for the company.
Following the comment of @jean, and considering the shadows on the seats, it may be taken during daylight (in flight or on the ground) but for sure not during night.

Also: I'm not sure at all about the seats but they look like old Aircelle seats (sth like 15 years ago) AirFrance was using in their planes. Not put this guess in my answer, I doubt we can get sth from it.

Looks like this Avianca cabin interior https://www.youtube.com/watch?v=X8ic-XnWUb8

@Florian You're right, it seems to be like this one. Yet I am not sure as this type of plane is owned by several other companies for which I haven't found the cabine interior plan. But I can't find a detail in your videao that don't match the picture.

I looked up from the other carriers that are listed in WP:EN some cabin interior pictures on google image search, they all look totally different.

@Florian Thank you for this info, post updated. Next step is to look on flightradar24 or similar website, if anyone has an account and enough time for this.

I tried but since the time stamp doesn’t have a time zone it’s rather difficult to tell which one it could have been

Yes, same pattern and curtains of an avianca, just guess it's a A319 based in seat configuration

@jean A319 at Avianca has 5 rows between exit door and curtains, and I think there are fewer rows here.

@Bentoy13 Looking again in the police site (better resolution) I can see the emergency exit. One question: is this day light from the windows? I ask because the time stamp

@jean I think you're right, I didn't notice the shadows. Interior light can't lighten the side of the seats. Another element to take into account is the fact that the top ligths appear saturated on the ceiling of the cabin, so the daylight may be not so strong. I think it's hard to guess the actual hour, the only thing we can be confident enough with is that the picture must be taken not during night (all people there seem awake, I can't see any sleep mask).


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Is Ducted fan or Propeller better to maximize forward speed of RC aircraft with a given power?
I'm designing an RC aircraft of which the final goal is a crazy top speed. I'll be 3D printing and sanding / finishing the air frame surfaces to reduce drag. In order to achieve maximum top speed, should I use open 5 inch diameter 3 blade props (the ones used in racing drones) with a large pitch and beefy motor, or two (60-80mm) ducted fan (EDF) units? Power draw is not a limiting factor here. Also, should I go for a larger EDF than 80mm? (wingspan is about 70 - 80 cm, total mass of about 700-1000g) . I'm aware that EDFs induce a greater delta V on the air they move, but does this actually mean I can expect a larger top speed?
[edit:] I could also go for a single 100mm unit.

related: How much thrust is gained from placing a propeller in a duct? and Why don't drones use ducted fans?

According to my understanding, the general rule is that, after taking into account pitch and everything else, if you can accommodate a prop with a diameter that is larger than the largest duct you can accommodate, go with the prop. If the largest prop you can accommodate is the same diameter as the largest duct you can accommodate, go with the duct. In other words, only go with the duct if you can't accommodate a prop of larger diameter. A duct doesn't beat our a larger prop, it only helps make up for a smaller one.

RPMs for models are typically 3x higher than full-scale aircraft, allowing props to hang in there with all their benefits at higher airspeeds.  Few models are flown at 30,000 feet or at airspeeds greater than 200 knots.

A propeller is better. Just look at any pylon race model to see what they use.
DKNguyen put it very nicely:

A duct doesn't beat a larger prop, it only helps make up for a smaller one.

You're right that "EDFs induce a greater delta V on the air they move" but EDF models generally use twice as much power as a similar aircraft with a prop. Moving half as much air twice as fast generates the same thrust but requires twice as much power. This is because momentum is mass x speed, but energy is mass x speed squared.
Ducts (and multi-blade propellers) are used when you don't have space for the right sized prop for the engine power.

Like turbo fans, the idea of EDFs is to have higher air speed in a smaller dimeter compared to a propeller. The problem is you need 4x the air speed when halving the diameter to produce the same thrust. EDFs are typically even smaller diameter than half a propeller.
To get the same thrust the rotational speed of the EDF is higher than a typical electric motor so gearing up would have to be achieved.
Then the top speed of the fan would be supersonic which introduces stresses on the blade and vibration and noise. So steel blades would be needed.
The upside is the max speed would be a lot higher than a prop because the pitch x rpm would be faster than any propeller.
The result would be an RC that would not have prop stall at take off and a higher top speed than a prop can achieve.


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What is the aspect ratio of T-tail for common twin turboprops?
I'm trying to figure out typical AR values for a T-tail configuration (at the conseptual design phase). Working with Raymer I found this table:

I can see the typical AR values for the vertical tail but I cant find anything about the horizontal one. The only info that I got is that it should be lower than wing's AR. The aircraft that I'm interested about is a twin turboprop.

I'm not sure if JAWA has tail-aspect ratios. You could also find planform images of the aircraft you have in mind and calculate the aspect ratios. Here's a list of T-tail aircraft

Hmmmm.

[Source]

ATR42=6:1

[Source]

Dash8Q400=4:1

I'm not aware of any reason why the aspect ratio of the horizontal stabilizer of a T-tail needs to be any different than that of a conventional tail. I always interpreted the dash in Raymer to mean that the h-stab should be sized similarly in T-Tail as in a conventional. Apparently this source shares my interpretation.
Due to the end-plating effect, the vertical tail in a T-tail is more aerodynamically effective, resulting in generally shorter v-stab. However, for the same surface area, T-tail would have a heavier structure to cater for uneven load cases on the h-stab and the need for a bullet fairing.


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Why does Lift and Drag calculation over an airfoil ignore shear stress?
why in Lift and Drag calculation over an airfoil we ignore shear stress effect and focus on pressure distribution?

Shear stresses are important for drag. For lift their effect are two orders of magnitude smaller than pressure.


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How much the Concorde secondary nozzle contribute to thrust?
Does anybody knows how much did secondary nozzles bucket type on the Concorde's engines contributed to the increase of thrust during take of and sub-sonic flight? This is picture of the nozzles  and here is air flow during take of  all images from this link https://www.heritageconcorde.com/variable-exhaust-nozzles  source states that secondary nozzles reduces noise, as well as improved engine performance. I suppose performance means thrust?

I added diagram to my question.

This source suggests at subsonic speeds, the secondary reduces drag but doesn't contribute significantly to thrust.

At all times below Mach 0.55 – on the ground, take-off and initial
  climb – the buckets are set to 21 deg., forming a ‘jet-pump’ gap at
  the nacelle trailing-edge. The high speed jet exiting from the primary
  nozzle draws-in atmosphere air from above and below the nacelle to
  eradicate a source of base drag that would otherwise exit between the
  nozzles.


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Can the VNAV Path angle be modified in a Boeing 737 NG?
In the Boeing 737 NG series, can the usual 3 degrees Path angle used for VNAV Descent be modified through the FMC to another angle?

3 degrees is not usual for the path angle during the VNAV descent. It is the usual glide path angle on most instrument approaches.

In the 737 NG, the descent angle is never entered by the pilot into the FMC. One enters a Cost Index, or alternatively a descent speed that's desired, and the FMC, considering forecast winds, computes the Top of Descent point and the angles that it expects to fly in order to achieve that speed.
But, it never tells the pilot what descent angle it's planning to use (although you can see the present vertical angle to a specified waypoint/altitude). Whatever the angle it's planning, you can steepen it or shallow it by increasing or decreasing (respectively) the planned descent speed or Cost Index.
But, unlike some other FMC's, the 737's never explicitly tells you the planned descent angle for enroute operations.
(On an approach, it's a different story; there your gradient path is coded with the waypoints, and it is displayed on the LEGS page. But the user can't change values in the database like that. Speeds, yes; GP angles, no. A pure visual approach, with only a runway-end waypoint is the ONLY time you can specify whatever angle you wish... but that's only to the runway itself, and not at all usable on any other leg.)

I am type-rated on the 737-800 and I have never used  the FMC to program a specific angle that I want it to fly. The most economical way to descent is to use your VNAV profile that has been calculated (provided that it has necessary information like Descent Winds, speed / altitude restrictions and E/D point). So by selecting an instrument arrival procedure, it automatically generates an E/D point.
As from the FCOM 11.31.29 , a vertical angle can be assigned to approaches, approach transitions and STARs.
Ralph is right on the time when you can only change it with a visual approach.
The only time I have ever used it to set a particular Flight Path Angle is during a visual circuit maneuver where I would enter the FPA of 3 degrees in the approach procedure page when I enter the procedure preference in the DEP/ARR page.So example, when you pick a particular runway for the visual maneuver, it has some blanks for you to fill in the FPA and the runway ext. point.


text:
Do aircraft obtain better milage by flying high or by using "ground effect?"
It has been said the most fuel efficient way to fly is by flying in thinner air. On the other hand the largest plane in the world was the Russian "Caspian Sea Monster" Ekranoplan, which is said to be the most efficient plane for its weight.

Which method of flying is the most efficient and by how much when it come to time and fuel?

related (if not a dupe): Could an airliner be as efficient lower and slower? and Can ekranoplans (GEV) be more efficient than traditional airliners?

There really isn’t any strict single rule on whether an aircraft should fly high or use ground effect to increase endurance. It all depends on the design requirements and the purpose of the aircraft.
For cargo type aircrafts, it would make sense to use ground effect and fly over the sea, since speed is not really a major concern.
For reconnaissance aircraft, using ground effect is obviously impossible. For that they have to fly high and the optimum altitude would depend on aircraft parameters, and be a strong function of weight and possible induced drag.


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How can the fuel decrease evenly from both wing tanks when an A320 is running on APU only?
I stumbled over this sentence in a training material about fuel leak:
"If the fuel quantity symmetrically decrease in both wing tanks and the fuel quantity in the center tank decrease, the fuel leak comes from the center tank or the APU feed line."
Regarding the APU feedline part, what I don't understand is if the APU is feeding from the left tank always regardless if it's by the fuel pumps or it's own pump (in case the fuel pumps are off) how can the fuel decrease evenly from both wing tanks?

Related: Where is the APU fuel pump on the A320?

If you look at the fuel leak checklist you will see that you will come to this point only after you have confirmed that there are no leaks from the inner wing tanks and engine fuel feed lines.
According to the checklist if there are no visual indications of fuel leaking near the engines, you are to ensure the fuel cross feed is maintained closed. Then you turn off the center tank pumps. When the center pumps are off the engines will be only supplied by the inner tanks. Then you are to monitor the fuel in the tanks for a total of 30 minutes. If one tanks depletes faster than the other tank by 300 kg you can assume an engine leak. So, you shut off the associated engine. If this stops the leak, engine leak is confirmed. If it continues to leak you can assume an inner wing tank leak.
Now, the checklist says, if both the inner tanks deplete at a similar rate, you can suspect a center tank leak (remember the pumps are off) or a leak from the APU feed line. I have highlighted the word similar, because the checklist says to check for similar readings, not same or symmetrical readings. Yes, with the APU on, the left wing inner tank will deplete faster than the right tank, but it should not bring a massive difference to the figures. The A320 APU burns about 130 kg of fuel per hour. The checklist says to monitor the fuel for 30 minutes. This means, at the APU fuek line a leak would happen at a lower rate. That is why it says to look for similar rates, because an APU fuel line leak is quite insignificant. But you cannot still assume no leaks at the APU fuel line. The fuel leak checklist is a very conservative checklist. It is all about trying to close off any possible areas where a leak might be possible.
The checklist also says that if you have a fuel smell in the cabin, to turn off the APU to prevent additional fuel loss. It uses the word additional because the rate of leak at APU fuel is insignificantly low.

The checklist says similar not symmetrical (highlighted).

I have had a situation like this while on ground during a long transit halt. As mentioned in your question how can fuel decrease evenly from both tanks with all the fuel pumps off.
The answer is to open the cross feed valve. As with the fuel pumps off the APU has its own fuel pump which provides feed only from the left hand side fuel tank. Keeping the cross feed open will negate this as fuel can now be fed to the APU feed line from the right tank also. Refer the below diagram
The fuel cross feed valve can also while on ground be used to transfer fuel from one tank to the other. Say in this case for example if we want to transfer fuel from the right tank to the left tank we have to follow the below procedure.
On the refuel control panel we have to put the mode selector to defuel/xfr position to open the defuel/xfr valve this in turn connects it to the refuel gallery. Post this the recieving sides refuel valve has to be opened and the non recieving sides refuel valve has to be closed. Once this is finished we switch on the fuel pumps of the tank from which we want to transfer fuel and keep the other sides fuel pumps off. The same diagram above can be used for understanding.
However for a more detailed diagram refer to the attached diagram below.

From this diagram you can clearly see the APU feed line and it's fuel pump along with the cross feed valve. Now even thought the fuel pump is located in the centre tank it takes its feed from the left tank only. This is with the cross feed valve closed. The moment the cross feed valve is opened the APU feed line is now also supplied by the right tank.
Hope this clarifies

Just out of curiosity.  Is it actually called a cross-feed valve or a cross-flow valve.  In my mind, a cross-feed system takes fuel from a different tank than normal and directly supplies it to the engine.  In other words, the fuel doesn't flow from one tank to another for the engines to burn it... that would be a cross-flow system.

@wbeard52, the diagram (see the linked question) calls it a cross-feed valve and it does seem to be a cross-feed valve, connecting the left and right feed lines, not the tanks themselves. I am not sure how it works for transfer (either it can flow back through the pumps when they are off, or makes a detour through the engines and return lines)

@JanHudec as far as the system function goes it's very simple you need some form of pressure to move the fuel from either sides tanks while on ground with the engines shut off. So in this case if we want to transfer fuel for example from the right tank to the left tank we will first open the cross feed valve and then switch on the right hand side tanks fuel pumps on. While running the pumps will now provide sufficient fuel pressure to allow the fuel to flow through the cross feed valve to other sides tanks. I have also had this situation while on ground during the refueling. We followed this.

@Jai, but how does the fuel actually get into the tank when you do that? The cross-feed valve connects to the feed line, which is connected to the tanks via the pumps on both sides, so the fuel can't flow directly into the tank. Either it has to be able to go back through the pumps when they are off, or flow around the engine and through the return lines.

@JanHudec I am not sure how Jai did it. I hope he gives us a more detailed answer. The time I had to do it, it was a bit more tedious. You have to first turn on the wing tank pumps (the side you want to transfer fuel from), and then turn on cross feed. Then you have to go outside and on the refuel panel, set the refuel mode selector to Defuel/ XFR and then the refuel valve of the receiving tank to on. That should start the transfer. And you are right, the X feed is in the engine supply line. It cannot transfer fuel between tanks as far as my knowledge is concerned.

@AnasMaaz you have addressed the fuel leak checklist. I am well aware of this procedure and what the FCTM also says. I have specifically addressed the doubt of the question as to how can fuel decrease evenly from both fuel tanks when only the APU is running.

Also the procedure explained by you is the whole procedure as mentioned in the FCOM. Which is correct I only explained what system controls were manipulated while in the cockpit.

@jai I am talking about your comment on how fuel is transferred in cockpit. Regarding the main question, I think OP asked why specifically the fuel burn is same between inner tanks in the 'fuel leak' procedure. That is exactly what is in the question. And I answered that.

And I am still very interested in knowing how you transferred fuel between tanks with X feed and tank pumps.

@AnasMaaz in terms of fuel transfer the entire procedure was followed as per the FCOM ground fuel transfer procedure. Again as i mentioned I only explained the cockpit control changes we did and how that helps. To actually move fuel in between tanks as mentioned by you the defuel/xfr position has to be selected. Once done that valve opens. After opening of that valve again as mentioned by you we shut the refuel valve of the non recieving tank and open the receiving tanks side. The pumps in turn provide the necessary pressure to move the fuel. The fuel flows through the refuel gallery.

@janhudec hope that answered your question. The fuel doesn't pass over the fuel pumps but goes through the refuel gallery to the open refuel valve the fuel inturn goes to the outer tank which spills over to the inner tank. I'm cass of the A321 there are only wing tanks and no outer and inner tanks

I believe your engineers operated the refuel panel then? That clears it up.

@AnasMaaz yes our company's engineers operated the refuel control panel and the mode selector positions.


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How do pilots align the HUD with their eyeballs?
In fighter jets, how do pilots align the HUD with their eyeballs?

A pilot will usually adjust his seat height to align his eyes roughly with the design eye-position in an aircraft. As there is almost no parallax error, even with the wrong seat height, HUD-elements will still align correctly with objects outside, however, some HUD-elements will be outside the projected field-of-view and not be visible to the pilot.
The HUD is an outgrowth of the reflector or optical gunsight developed at the end of WWI and which started to appear on most fighters by the mid 30s.  The secret to them is the collimator, or collimating lens, which is the big magnifying glass looking thing on a reflector sight.
A collimator aligns the light rays of an object to be parallel after they pass through the lens.  The light rays being parallel, if you look at an object through the lens, it will be focused at infinity (it will look like it's a few hundred feet in front) even though it's only inches away.  If you shine a light through a plate with a circular slot and hole in the middle, project the light off a mirror straight up, through the collimator, then reflect it back using a sloping sheet of glass (or the windshield), and you the pilot get in line with the projected light's rays, you will see an illuminated circle and dot (called a reticle) that looks like it's suspended in space several hundred feet in front.
You can only see the reticle if you are where the light rays are being projected, and there is almost no parallax error, so as long as you have the reticle in view, you are correctly lined up with a target up front.  If you move your head to the side or up or down, the image will start to disappear off the side; but there is a fair amount of leeway, and your eye alignment doesn't have to be perfect, just enough to keep the reticle in view.  This was so much easier than keeping your eye perfectly aligned with a ring and bead sight.
The effect is similar to looking at something projected on a movie screen at a drive in, while your car windshield is painted black except for a little square you can look through.  If your head's in the right spot, you can see the image on the screen out front, focused way out there, but if you move your head the image starts to disappear.
In the 60s instead of just projecting light through a plate with slots and cross hairs etc., they started to use CRTs and project more information, like radar indications, horizon indications and such.  By the 70s they where sophisticated enough to produce all the symbology you see on modern HUDS.
But in the end, a HUD is just super fancy version a WW2 gunsight.

The HUD's behavior is the same as that of a rifle scope: If you move your eye off center, that does not shift where the reticle is with respect to the target, but the target starts to move out of your field of view.  That makes me wonder if a similar principle is at play in a rifle scope as in a HUD (making the light rays be parallel).  Nice answer.

@WayneConrad: A reflex sight on a rifle works on the same principles as the fighter plane reflex sights described in this answer. Parallax compensation on a telescopic sight (a scope) works differently, if it's there at all - many scopes have no parallax compensation.

The other way around - you set up the pilot for eye-to-HUD alignment. When you are in the right position then you can see all the data-fields on the HUD - if you can't see them, you move your seat.

Pilot size and shape are also controlled unlike car drivers.

I can see how this works for the data fields but is it also sufficient for alignment of the display elements that are referred to the outside world? e.g. target reticles, flight path vector

@pericynthion most pilots only have one pair of eyeballs, so if they are aligned correctly for one thing, they will be aligned correctly for everything else.

@alephzero I don't follow - being in a position adequate to see the HUD indications doesn't necessarily mean that you will see them in the right angular location relative to the world outside the airplane.

@pericynthion Actually, it does. Say the plane knows the object it wants to cover is 15 degrees above straight in front of the plane. If the plane knows the position of the pilots eyeballs relative to the HUD, it can calculate the position on the HUD that is 15 degrees above straight in from of the pilot. Displaying on this spot will cause the object to be aligned with anything 15 degrees above straight in from of the plane. The math to do this is interesting and fairly simple, if your interested look into 3d projection.

Right but I don't think the plane does know the position of the pilot's eyeballs relative to the HUD. Or if it does, I think pointing that out and summarizing how it knows that is an essential component of the answer.

@pericynthion The plane does know where the pilot's eyes are, because the seat is adjustable, so the pilot can adjust the seat until their head is in the right position before takeoff. See also this video (this is for a commercial aircraft, but military aircraft have similar systems).

The HUD's projector has to stay aligned with the longitudinal axis. There are two options; move the pilot so the pilot's eyes are in optical alignment with the HUD, or move the HUD up or down without tilting it, that is, keeping the light projected by the HUD parallel to the longitudinal axis.  Easier to move the pilot's seat up and down.  Once you are are more or less in position, you can move your head a few inches up/down and left/right before the edges of the projected image start to go out of view.  The closer your face is to the reflector/combiner, the more leeway you have.

@alephzero there is however a thing called "two eyes". While I'm not sure if the technology is already used in jets, it is for drones; A good aligned projector can give a 3d view of objects giving more information at locations where it is useful (augmented reality).
However for this both cameras need to be aligned to the eyes (logical), but also the distance between your eyes influences "how" you see 3d objects; so that width needs to be used for calculations when displaying augmented reality. For drone pilots this is quite some difficulty.

Important point. HUDs are mechanized so that the "point" that the images appear appears to be several feet (I'm not exactly sure how far away it is, but it's at least several feet) in front of the pilot's eyes, far enough in front to eliminate the parallax effect that would cause inaccuracies as a result of the pilot's head being displaced left, right, up or down from an exact position. Indeed when you sit in the seat and look at the HUD as you move your head, the images appear to move in the combining glass as you are moving your head, so they remain on the same point in the distance.

When an airframer (whoever it happens to be) produces the requirements for a HUD, the following key optical items are specified.
1 The Eye motion box. This is a 3 dimensional zone and the critical items are the size of the box and the distance and angle from the HUD. This means that the pilot can move his head (not much in a fighter, clearly).
As noted in other answers, objects will come in and out of view as the viewing position is adjusted. That means there is an instantaneous field of view (the angles for which items can be seen from any point in the eye motion box) and a total field of view (which is everything that can be seen when viewed from all positions in the eye motion box).
2 The Cyclops eye position. This is the centre of vision of the pilot when viewing from the optical centre of the eye motion box.
There are many more parameters that I won't go into here as we are looking at where the pilot must be to properly view the HUD (although brightness range of the display can be very challenging as it might be in sunlight or in a moon-less night)
This leads to some interesting issues:
As fighters (and some other aircraft with HUDs such as the B2) are fitted with ejection seats, it requires the ejection seat manufacturer to specify the nominal height of the pilots eyes and the height adjustment range. Whether the position is specified by the ejection seat manufacturer or the airframer depends on whether the aircraft is new. (Oue newest, the F-35 does not have a HUD).
This is also one reason there are fewer female pilots in HUD equipped aircraft; their average height is less than that of males and most aircraft with HUDs today were designed around male height statistics.
A very expensive operation on a new aircraft is boresighting (aligning) the HUD and more specifically the mounting tray. This involves ensuring that where the view field of the HUD exists is precisely aligned in all 3 axes of the aircraft so that the display is indeed properly viewable from the eye motion box.
Because no mechanical alignment is ever perfect, the boresighting procedure actually generates a set of correction parameters that are used to adjust the location of items on the display; the method varies by HUD. This has to be performed on a per-aircraft basis.
Another interesting fact; as the surface of the HUD combiner is curved in some aircraft (F-22 and Typhoon comt to mind), any images must be pre-distorted so they appear not to be curved to the eyes of the pilot.
All this processing must be done with very low latency (16 millseconds is a common requirement) so modern HUDs have quite a bit of processing power within them.

HUDs are mechanized so that the "point" that the images appear appears to be several feet (I'm not exactly sure how far away it is, but it's at least several feet) in front of the pilot's eyes, far enough in front to eliminate the parallax effect that would cause inaccuracies as a result of the pilot's head being displaced left, right, up or down from an exact position. Indeed when you sit in the seat and look at the HUD as you move your head, the images appear to move in the combining glass as you are moving your head, so they remain on the same point in the distance.

In what way 'mechanized'? What point are you making different to John K's one about optical characteristics, which require no mechanisation to achieve this effect?

@PeteKirkham: Perhaps "mechanically designed" (or "built") is what Charles meant to say?  (And BTW, this is a repost of his comment on RAC's answer; perhaps written before reading John K's answer.)

Yes. Exactly. They are mechanized, constructed, designed and built, however you want to word it, so that the images you see "appear" to be several feet in front of the glass surface they are projected on.


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How to start up a Lockwood-Hiller Valveless Pulse Jet?
So me and my group mates in university got a lockwood hiller valveless pulsejet manufactured, and there was one error in manufacture. We are having trouble with trying to start the engine as demonstrated in the attached link. Is there an error in the engine you can see?
One manufacturing error we identified is that the exhaust cone is slightly at an angle and is not exactly parallel to the combustion chamber+intake which looks something like the attached. Could making it parallel solve it?
Also, would putting the gas cylinder upside down solve it?

I don't have experience with pulsejets, but yours has other obvious differences from easy-starting versions I've seen on YouTube.  Differences that seem more likely to affect starting and running than the angle of the exhaust.

so the major differences you may notice is that the angle of the exhaust pipe is incorrect, but that same exhaust pipe is also too long, compared to other pulse jets, is there anything else noticeable I might be missing? The combustion Chamber is slightly larger as well

What is the pipe that you're holding to the intake? In other videos people seem to use a leaf blower to generate the necessary air flow.

I am using 50 psi unregulated pneumatic power. The uni leaf blowers are all broken. With regulated pneumatic power the engine is not getting into that intermittent popping phase, just black smoke from exhaust, so we switched to unregulated pneumatic power. One thing to keep in mind that our pulsejet is large compared to other designs on youtube, and specifically the combustion chamber is also large, which means that maybe I’ll need to use more fuel?

The key to the operation of a valveless pulse jet is the oscillating shock wave created by repeated explosions in the combustion chamber. In this video they talk about using a spark plug to trigger these explosions initially, and once the engine is running the process is self sustaining.
Looking at your video I don't see any connections to the engine except the fuel pipe, and you seem to be attempting to start the engine by pushing some burning material into the combustion chamber. This will not create an explosion, so the shock wave that keeps the engine running won't be created either.

From my video watching, well made and tuned Lockwood-Hiller jets can usually be started (at least on propane) by a flame at the short end of the pipe, which propagates into the combustion chamber.

I’m using this ignited cloth because it seemed to work with some other material here: https://youtu.be/qwuDkOB9Jik  Also, colin furze (some youtuber) made a lockwood hiller pulsejet and one of the ways he started the engine was simply by igniting the intake with a torch, given that the gas is flowing out of the intake, which he recommends that the gas is flowing out of the intake initially, not the exhaust.

To my eye, comparing with memory of many YouTube videos, the conical ends of the combustion chamber are too short and the bends in the exhaust pipe are both too small radius and the pipe is slightly flattened at those bends.
The angle of the exhaust pipe shouldn't matter -- I've seen these build completely straight and gotten to run -- but the sharp bends will produce out of phase shock wave reflections, and the too-short ends on the combustion chamber may also have a negative effect on the resulting gas compression.

oh okay - your issue is with the tangents on the exhaust cone. This cone actually should have been rolled, but the fabricator had problems rolling it and their only solution was to create it out of tangents. I kid you not, I looked all over the country for a fabricator who can make the engine, and they were the only ones who can make it, let alone a reasonable price. The problem is our university is underequipped, and we simply do not have the time to buy the tools required and try to learn it. We do not have any rolling machines, welding equipment or laser cutters.

No, the tangents on the exhaust cone shouldn't matter.  I was referring to the right angle and near right angle bends in the pipe between the combustion chamber and the exhaust cone -- these look, to my eye, too sharp (bend radius too small) and appear to have some pipe flattening.

Update: I inverted the cylinder and the engine buzzed and pulsed, but I stepped off too early cause the exhaust explosion was too loud and I wasn’t expecting; I got scared basically. I was expecting yet another failure lol. Anyway, we tried starting it after that

Liquid propane delivers a lot more actual fuel than gas from the top of the tank.  You may also find it helps to use a water bath (not too warm, please) to keep the tank from chilling as you draw the propane from it.

oh so after that we tried to start it and it wont start and just blew a flame of wasted gas, as we checked the bottom of the combustion chamber it was freezing like you are saying. Is that a reason why it won't start anymore? When does that freezing happen, after engine is stopped? Or when it is running?

Also regarding the elbow joints, I agree with you, they are more towards a sudden turn than a smooth turn and I could use a larger radius of course, but we chose the small radius route. Some designs used the small radius route while others used the longer radius, and both worked. I honestly don't know the reasoning behind the shorter radius design, even though the longer radius should yield smoother flow. I could barely find anything to justify the radius ratios in different sections of the engine.

Update: the engine successfully started and self sustained with the inverted cylinder! Here: https://youtu.be/Sb-7KXpG0wE

Excellent!  Glad you got it running.

This "university" project may have made an old mistake of scaling up before the design parameters were properly set.
A pulse jet, theoretically, is a truly amazing engine that uses an invisible piston to compress the next batch of air and fuel, a resonant shock wave.  A properly shaped "tuned pipe" must be made or it simply will not run effectively.
So you may wish to start with a smaller, more reliable r/c model pulse jet as a demonstrator, rather than trying to get that monster to run.
Once experience is gained, move on to scale, keeping in mind the risk factors.
Tuned pipes also have application in increasing the output of 2 cycle engines by reducing losses of fuel and air out the exhaust port as the cylinder recharges for the next power stroke.

oh and i forgot to tell you as well.. the engine was also never meant to be that big, but the company also insisted on the larger size as it wasnt feasible for them to fabricate the small ones, they had some ready made larger sizes so we had to edit the design and readjust the parameters to whatever sizes they had of parts.

the design parameters we followed were the 1967 original patent of this engine, and the exhaust pipe was supposed to be by most 130cm long, but when we had issues in manufacturing we had to go bigger.. I’m serious manufacturing has been that much of an issue so far

@Ali Al Kayyali you all may learn much from the experience.

Just to answer my own question and not waste anybody’s time, all I needed to do was to invert the cylinder to get Liquid Gas to flow out. It worked! Here:


text:
what reference of airfoil is used to define lift and drag?
On aerofoil section, the force of lift acts perpendicular to, and the force drags acts parallel to the :
A: chord line
B: flight path
C: aerofoil section upper surface
D: longitudinal axis

Free stream flow. You should have a word with your instructor.

why are you hesitating between all the possibility? It recommend you read the airfoil section of how it flies?

I change the title of  your question so that it looks like a question. feel free to edit  again if it does not reflect what you wanted to know

@Jimmy The instructor expects B (flight path) as the right answer. Which is correct if there is no wind.

@PeterKampf Flight path is inertial. It has no bearing on aerodynamics. Even if we are talking about a wind tunnel case where the section is constrained, flight path is very poor wording; pitch attitude would be marginally better in that case.

@Jimmy: So what? The air flows towards the aircraft in exactly the opposite direction to the direction of the aircraft's movement. That is what counts.

@PeterKämpf Not sure what you mean. For a wind-less relationship, $\alpha=\theta - \gamma$. $\gamma$ being the flight path angle. $\alpha$ is what counts here. See https://en.wikipedia.org/wiki/Flight_dynamics_(fixed-wing_aircraft)#Short-period_pitch_oscillation for illustration.

@PeterKämpf is correct, there should be an answer about the relative wind. He is also correct that the answer should be "B", we apply the CASA approach which is "the least incorrect" response.

@PeterKämpf, why does pitch angle have to be zero?   You could be pitched 45 degrees nose low in a dive and drag will be opposite to, and parallel with, your flight path.  What am I missing?

P.S.  Where were the homework police on this one?!

@MichaelHall I don't know why I said this. Lift and drag are defined in the aerodynamic system, and this is in turn defined by the direction of flow. Pitch angle could be anything.

So @JZYL, what point are you trying to make in asserting above that pitch attitude is a "marginally better" term, (than flight path) in describing the vector opposite to drag?

@MichaelHall The main takeaway is the choices are all wrong.

@JZYL, that isn't helpful in the least.  If you think you can clear this up then please provide an answer and not murky criticism.  (keep in mind the context of the question)

This is a basic question, and I give you the answer from the initial pages of a aerodynamic learning textbook.
The  is presented in next figure:

Where:

L: lift: component of R (resultant force) perpendicular to air stream,
D: drag: component of R parallel with the air stream,
N: Normal Force: component of R perpendicular to chord,
A: Axial Force: component of R parallel with chord,

For further information you can refer to Anderson's Aerodynamic book.

Good diagram.  This is very interesting as it presents a realistic representation of the forward component of lift (relative to perpendicular of wing) created by airflow over the leading edge curve at a higher AOA.  This was noticed in drag studies, producing a negative drag coefficient!  This is also a great illustration of areodynamic center through one point as AOA (potentially) changes.  Correct answer - B.